1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display (LCD) device, and more particularly, to a backlight unit capable of enhancing color uniformity, an LCD device having the same, and a method for providing substantially white light for a liquid crystal display device.
2. Description of the Background Art
Generally, a liquid crystal display (LCD) device comprises a liquid crystal display (LCD) panel having a thin film transistor (TFT) array substrate, a color filter substrate facing the TFT array substrate, and a liquid crystal layer interposed between the two substrates; a driving unit for driving the LCD panel; and a backlight unit for supplying light to the LCD panel.
On the TFT array substrate, a plurality of data lines and a plurality of gate lines arranged in a horizontal direction cross each other to define pixels. The pixels are arranged as a matrix form.
On the color filter substrate, R, G, and B color filter layers are arranged at positions corresponding to the pixels. A black matrix for preventing light from being leaked between the color filter layers and for preventing color interference of light that passes through the pixels is formed on the color filter substrate.
A common electrode and a pixel electrode are respectively formed at inner surfaces of the color filter substrate and the TFT array substrate facing each other, thereby applying an electric field to the liquid crystal layer. The pixel electrode is formed on the TFT array substrate at a position corresponding to each pixel. The common electrode is integrally formed on an entire surface of the color filter substrate. Therefore, a voltage to be applied to the pixel electrode is controlled under a state that a voltage has been applied to the common electrode, thereby changing an alignment state of liquid crystal molecules of the liquid crystal layer and thus individually controlling an optical transmittance of each of the pixels.
The backlight unit supplies light to the LCD device that does not spontaneously emit light. When the light emitted from the backlight unit passes through the liquid crystal layer, an optical transmittance is determined by the alignment state of the liquid crystal molecules and thus an image is displayed.
The backlight unit is divided into an edge type and a direct type according to a position of a lamp to be used as an optical source.
In the edge type backlight unit, a lamp is positioned at one side surface or two side surfaces of an LCD panel, and the light emitted from the lamp is transmitted to an entire surface of the LCD panel by a light guiding plate.
The direct type backlight unit has been developed since a size of the LCD panel is increased to be more than 20 inches. In the direct type backlight unit, a plurality of fluorescent lamps are arranged at a lower surface of a diffusion plate in series thereby to directly irradiate the light to an entire surface of the LCD panel. The direct type backlight unit provides higher optical efficiency than the edge type backlight unit, thereby being mainly used to an LCD device of a large screen requiring higher brightness.
A cold cathode fluorescent lamp (CCFL), a hot cathode fluorescent lamp (HCFL), an electro-luminescence (EL) lamp, a light emitting diode (LED), etc. are used as an optical source. Among the optical sources, the CCFL and the LED are mainly used.
The CCFL provides high brightness and enhances brightness uniformity. However, the CCFL cannot provide a partial brightness control and an original color reproduction.
In the LED, R, G, and B LEDs are arranged in a stripe form thereby to provide a partial brightness control and an original color reproduction. However, the LED has inferior color uniformity to the CCFL. That is, since the LED emits white light by mixing R, G, and B light emitted from the R, G, and B LEDs, a pure white image can not be displayed and uneven brightness is caused when the R, G, B colors are not uniformly mixed together.